1. Field of the Invention
The present invention relates to outboard motors, and more specifically, to an outboard motor including a drive shaft extending downward from an engine and a forward/backward switching mechanism arranged to transmit drive shaft rotation to a propeller shaft.
2. Description of the Related Art
As described in JP-A 2008-31868, for example, conventional outboard motors have a drive shaft extending in an up-down direction for transmitting power from an engine which is located above the water level to a propeller which is located below the water level. The engine is mounted on an upper portion of the outboard motor in such a way that the crank shaft orientation is in the up-down direction.
The crank shaft has a lower end portion connected with an upper end portion of the drive shaft so that they rotate together. The drive shaft has a lower end portion connected, via a forward/backward switching mechanism, with a propeller shaft which extends in a fore-aft direction of the outboard motor. The propeller shaft has a rear end portion provided with a propeller for integral rotation therewith.
The forward/backward switching mechanism includes a driving bevel gear attached to a lower end portion of the drive shaft; a pair of driven bevel gears rotatably supported by the propeller shaft for engagement with the driving bevel gear; and a dog clutch which serves as a switching mechanism to turn on and off power transmission from the driven bevel gears to the propeller shaft. The driven bevel gears sandwich the driving bevel gear from front and back sides of the outboard motor, thereby making engagement with the driving bevel gear.
The dog clutch has a slider which rotates integrally with the propeller shaft between the driven bevel gears, and includes teeth provided at two end portions of the slider and teeth provided in each of the driven bevel gears. The conventional forward/backward switching mechanism can switch between a forward driving state where the teeth on one side of the slider are in engagement with the teeth on one of the driven bevel gears; a rearward driving state where the teeth on the other side of the slider are in engagement with the teeth on the other of the driven bevel gears; and a neutral state where none of the slider teeth are in engagement with any of the teeth of the driven bevel gears.
A problem in the conventional outboard motor described above is a knocking sound which is generated by the forward/backward switching mechanism repeatedly performing switching in a certain periodic cycle when the engine is operating at a constantly low rotation speed such as during trolling. Hereinafter, this knocking sound will be called trolling noise.
The cause of the trolling noise is believed to be mutual hitting of components used in the power transmission mechanism. For this reason, there is a requirement in the conventional outboard motor that the trolling noise should be eliminated so that the components will not be worn by the mutual hitting.
The trolling noise is believed to be caused by mutual resonance of power transmission components which are utilized in the transmission mechanism ranging from the drive shaft to the propeller shaft. Specifically, the trolling noise is believed to be caused in the following sequence: in the dog clutch of the forward/backward switching mechanism, the slider teeth and the driven bevel gear teeth are in engagement with each other during the trolling operation, but become momentarily disengaged by the resonance, and then come back to a strong engagement right after the momentary disengagement.
It should be noted here that as a related art in this field, JP-A SHO 60-215495 discloses a propeller rotation smoothing apparatus for a marine vessel propelling system. In the apparatus, a first transmission section which has a relatively small torsion resistance and a second transmission section which has a relatively large torsion resistance are provided between an engine-side drive shaft and the propeller-side drive shaft. In this arrangement, the engine-side drive shaft and the propeller-side drive shaft are coupled with each other by the first transmission portion for integral rotation with each other. The engine-side drive shaft and the propeller-side drive shaft are designed to perform predetermined relative rotation, and only after that they become coupled by the second transmission portion for integral rotation with each other. However, this apparatus cannot reduce the trolling noise.